The desire to maintain a youthful appearance by preventing or reducing wrinkles in the skin is an important issue in human society. Many techniques have been designed to achieve the above issue. One of the techniques, known from the published international patent application WO 2008/001284 A2, is to create a focal spot in a dermis layer of the skin to be treated. Said WO application discloses a skin treatment device with a laser source and focusing optics, wherein the power of the laser is selected such that Laser Induced Optical Breakdown (LIOB) affects the skin in order to stimulate re-growth of skin tissue and reduce wrinkles. This LIOB is based on strong non-linear absorption of the laser light by the skin tissue, which occurs above a certain threshold value for the power density of the laser light. This strong absorption causes localized plasma that is able to damage or even remove tissue at the location of said plasma. This is caused by secondary, primarily mechanical effects such as rapid expansion of the generated plasma. This effect is very local, because below the threshold there is zero or very little linear and non-linear absorption, while above the threshold plasma is generated, which even more strongly absorbs the radiation. In other words, effects such as LIOB only occur at the focal spot, while above and below the focal spot no or very much weaker effects occur. This means that for example the epidermis may easily be safeguarded against undesired effects or damage.
The focal point is created at a fixed treatment depth, somewhere between 0 and 2.0 mm. This depth is selected based on the typical composition of human skin. In some cases, however, the optimum treatment depth may be different. The optimum treatment depth depends on, e.g., the thickness of the stratum corneum and the epidermis.
In addition to LIOB, also other multi-photon absorption processes such as second harmonic generation, third harmonic generation and other higher harmonic generation processes may be used to image and modify tissue to stimulate re-growth of the modified tissue to rejuvenate the tissue and reduce wrinkles Laser skin ablation through multi-photon ionization (e.g.: laser induced optical breakdown) requires high light intensities in the order of 1013 W/cm2. Due to a very high photon flux (typically >1031 cm−2s−1), multiple (N) photons with energy of hv at a wavelength of λ behave like a photon of energy Nhv interacting with an electron to free it from the valence band. This requires that the total energy of the absorbed photons is greater than the ionization potential (Nhv>Δ). The generation of this seed electron by ionization requires multiple photons (N) having the same polarization confined in space (focal volume) and time (˜femto seconds) with a total energy exceeding the ionization potential (Nhv>Δ) of the material. Multi-photon ionization deep inside skin is challenging to achieve.